Won't happen. If Apple added Swift as a sibling language alongside JavaScript, most people would still use JavaScript because of its wider support. If Apple replaced JavaScript by Swift, that would be suicide for their platform.

What, IMO, will happen is that WebAssembly will replace JavaScript, creating a level playing field for all languages. JavaScript will no longer be the only language you can on both server and client, so we will see all-Python, all-Javascript, all-Java, all-whatever frameworks in the browser (potentially each with its own GUI library, but I think that won't happen). All-Swift could be something that Apple could provide, and chances are they are already working on it.

Haha!